The invention relates, generally, to a system for testing for ISDN NT1-U interfaces in which energy is applied to the interface and voltage measurements are taken across the interface at predetermined times corresponding to points on the NT1-U dc characteristic curve.
It will be appreciated that telecommunication service providers must efficiently service and maintain the network in order to provide reliable, high quality service. One of the difficulties faced by the service providers is in identifying the source of a problem in response to a customer's complaint. As a result, numerous test systems have been developed for testing the operability of various components in the network to determine if the problem resides in the customer premise equipment (CPE), in the line drop connecting the CPE to the switching system or elsewhere in the system. As will be appreciated, the source of the problem dictates who is responsible for its repair and how the repair is made. By quickly and economically identifying the source of problem, resources can be more efficiently allocated.
To explain the environment in which such testing systems operate, the arrangement of a typical switching system will be briefly described. A typical switching system, illustrated in FIG. 1, includes a central office 2 consisting of a switch such as the 5ESS switch manufactured by AT&T and described in U.S. Pat. No. 4,592,048 issued to Beckner et al. on May 27, 1986. The central office 2 is connected directly to a large number of CPE's 4 over subscriber lines 6 and is connected to other switches in the public switched telephone network 8 via trunks 10 such that calls can be muted between CPEs 4 and other CPEs in network 8 as is well known. The central office 2 includes processing and switching capabilities for routing calls through the network, performing administrative and billing functions, providing special services such as conference calling, call forwarding or the like, and for running peripheral equipment. A central office is a large, sophisticated piece of equipment capable of hosting tens of thousands of subscriber lines.
Each of the subscriber lines 6 is constructed of a pair of relatively expensive copper wires having a usable range of approximately five miles. Beyond this distance, the signal quality degrades. To limit the length of the subscriber lines and to minimize the number of subscriber lines emanating from the central office, a relatively small and simple remote terminal 12 has been developed. One example of such remote terminals is the SLC family of terminals manufactured by AT&T and capable of hosting 2,000 subscriber lines 14 that serve CPEs 16. These remote terminals are typically located two to five miles from the central office 2 and are connected to the central office by a fiber optic cable 18, although the distance from the central office can vary. Such remote terminals include processors capable of performing call routing functions but not providing the administrative functions of a central office.
The advantage of such remote terminals is that telephony service can be provided in rapidly growing areas or in remote areas quickly and inexpensively by periodically adding remote terminals to an existing central office. Because a single fiber optic cable 18 connects the central office 2 to the remote switch 12, it is not necessary to run expensive copper wire from the central office to each CPE 16. As a result, the cost of extending service to new areas is lower. Moreover, there is very little signal degradation over the fiber optic cable.
There have also been developed even smaller remote terminals 22 such as the Multi-Services Distant Terminal (MSDT) manufactured by AT&T. These small remote terminals 22 are typically connected to one of the SLC family of terminals or other larger remote terminal 12 by a fiber optic cable 24. The MSDT can host approximately 20-30 CPEs 26 via copper subscriber lines 28. These small remote terminals are typically used to provide telephony services to the most distant and lightly populated areas such as in particular a remotely located building.
To test for problems in the system, many central offices 2 are provided with (or can be connected to) a local line test unit 3 that can be selectively connected to one of the lines 6 through the test access switch 7 for testing the customer service lines 6 and CPEs 4 directly from the central office. It is also known in the art to provide centralized test unit facilities 9 remote from the central offices that access a plurality of central offices via wire or fiber 11 to test subscriber lines and CPE's served by those central offices. Such test facilities, while providing sophisticated test capabilities, are relatively expensive such that their use is justified only for large switches, such as central offices, that host a large number of customer service lines.
Many of the larger remote terminals 12 include integrated test units 13 for testing the subscriber lines 14 and CPEs 16 connected to those terminals. These test units are less expensive than either the local or centralized test unit equipment associated with the central offices such that their use is economically justified for the fewer subscriber lines served by the remote terminals, although these test units do not have all of the test capabilities of the more expensive test systems. Typically, the test units 13 in the remote terminals are controlled by the central office 2, either independently or in concert with the centralized test unit 9, and perform tests based on requests from the central office. The test units communicate with the central office either over fiber optic cable 18 or over a twisted wire pair 28 provided specifically for transmitting test signals.
Finally, it is known to provide a channel and drop test unit (CDTU) 21 that includes a simple test circuit in the small remote terminals (such as the MSDT) 22 that can determine if an analog telephone is connected to the subscriber line and is operating. One such test circuit is the AUA411 manufactured and sold by AT&T for use in the MSDT remote terminal and described in detail in U.S. Pat. No. 5,054,050 issued to Burke, et al. on Oct. 1, 1991. While the described circuit is relatively inexpensive, it has been used only to perform a method for testing for analog service.
It will be appreciated that in addition to analog service, many businesses and homes now receive ISDN services. ISDN customer premise equipment (CPE) communicate with ISDN switching systems in two 64 kilobits per second (Kbps) B-channels and in one 16 Kbps D-channel. Each of the B-channels is usable to convey digitized voice samples at the rate of 8,000 8-bit samples per second and data at the rate of 64 Kbps. The D-channel is used both to convey signaling packets to effect message signaling between ISDN stations and switches or other ISDN stations and to convey data packets among ISDN stations. ISDN provides end-to-end digital connectivity to transmit voice, audio, visual and data information through user-network interfaces. The interface for transmission between ISDN CPEs and an ISDN switching system is a digital subscriber line. One common interface is the NT1-U interface which provides ISDN service using a two-wire digital subscriber line.
The operating specifications for the NT1-U interface are defined in the NT1-U dc characteristic curve put forth by the American National Standards Institute (ANSI) in the ANSI, T1.601-1992 standards. The testing criteria for the NT1-U is also defined by ANSI and Bellcore for the ISDN NT1-U dc characteristic which suggest measuring current flow for determining the presence of an ISDN NT1-U interface. The existing test circuitry used in the small remote terminals, for example AT&T's AUA411 circuitry, does not measure current flow. Rather, it measures voltages across the interface to detect analog service. Thus, it is not readily apparent that this circuitry is compatible with the suggested current flow test procedure set forth by ANSI. Moreover, existing ISDN interface tests, such as that used in either the local or centralized line test units, are too expensive for their use to be justified with the 20-30 lines served by the small remote terminals.
Thus, a problem in the art exists in that a simple, cost effective method for testing ISDN interfaces by measuring voltages across the interface has not been developed.